Gas burners having radiant burner elements have long been used to heat fluids in commercial, industrial and residential applications. Such applications include areas as diverse as home heating and commercial deep fat fry cookers. In one known form of gas burner, a combustible gas or air-gas mixture is passed through a burner element including one or more flat plenum clay tiles. The gas is burned on the surface of the element. Such burners have sometimes been found to be undesirable for many purposes, because they are inefficient as they often loose heat to the environment and undesirably heat incorrect sections of the apparatus in which they are employed. U.S. Pat. No. 4,919,609 (Sarkisian et al., Apr. 24, 1990) and U.S. Pat. No. 4,397,299 (Taylor et al., Aug. 9, 1983) disclose two examples of gas burners employing gas-permeable tiles.
It has also been found that, under some circumstances, gas burners more efficiently consume the fuel gas when their burner elements are configured as cylinders. While various methods have been employed to construct cylindrical burner elements, their use has been found to require a careful balance of pressurized gases to ensure that the supplied pressure is uniform, so that the flame on the surface of the burner element does not creep back into the burner, particularly into any mixing chamber contained within the burner element, and explode or backflash.
The following criteria are believed to be important in selecting the material to be used for distributing gas in a gas burner element:
1. The material needs to permit a low pressure drop across the burner element.
2. The material must have uniform openings for evenly distributing the gas mixture at the surface of the burner element.
3. The material must have good insulative properties in order to prevent backflashing.
4. The material must support ignition and combustion only on its downstream surface, typically its outer surface. This criterion is particularly important when the combustion gas is propane. Propane gas has a higher flame velocity than natural gas (about 2.85 feet per second to about 1 foot per second), so that its flame has a higher tendency to creep into the pores of a burner element.
Some prior attempts to meet these criteria have involved the use in burner elements of ceramic fibers in various configurations, such as in felts, sintered webs, random orientations and ceramic fiber filters. Such attempts often encountered drawbacks such as non-uniform pore sizes, high backpressures, and backflashing when flames crept back into the mixing chamber or other source of the combustion gas. The last-mentioned type of ceramic fiber burner element, the filter, is conventionally made by placing short, wetted ceramic fibers, and (optionally) a binder, over a screen of a particular mesh size, and vacuuming out the moisture to form a cylinder of fibers. Such a construction is subject to several of its own particular drawbacks. Such fibrous elements lack integrity. Moreover, because the fiber matrix must be thin enough to allow gas to pass through it, the strength of the matrix is compromised and the material degrades during use. Furthermore, with both filter-like constructions and constructions such as felts, the burner elements easily clog with dust and other impurities carried by the air and combustion gas, so that the filters require increased pressures to insure an adequate flow of combustion gas through the elements in which they are employed.
Felt-type ceramic fiber burner elements are shown in U.S. Pat. No. 4,604,054 (Smith, Aug. 5, 1986), U.S. Pat. No. 3,425,675 (Twine, Feb. 4, 1969), U.S. Pat. No. 3,208,247 (Weil et al., Sep. 28, 1965) and U.S. Pat. No. 3,191,659 (Weiss, Jun. 29, 1965). Burner elements constructed from vacuum-drawn ceramic fibers are disclosed in U.S. Pat. No. 4,883,423 (Holowczenko, Nov. 28, 1989), U.S. Pat. No. 4,809,672 (Kendall et al., Mar. 7, 1989), U.S. Pat. No. 4,746,287 (Lannutti, May 24, 1988), U.S. Pat. No. 3,275,497 (Weiss et al., Sep. 27, 1966), and U.S. Pat. No. 3,179,156 (Weiss et al., Apr. 20, 1965). A burner element incorporating sintered reticulated ceramic webs is shown in U.S. Pat. No. 4,568,595 (Morris, Feb. 4, 1986), while U.S. Pat. No. 4,519,770 (Kesselring et al., May 28, 1985) and U.S. Pat. No. 4,416,618 (Smith, Nov. 22, 1983) disclose burner elements including ceramic fibers in random orientations. Other burner element constructions are shown in U.S. Pat. No. 4,898,151 (Luebke et al., Feb. 6, 1990) and U.S. Pat. No. 3,726,633 (Vasilakis et al., Apr. 10, 1973), as well as in Japanese published applications JP 61-143613 (NGK lnsulators Ltd., published Dec. 18, 1984) and JP 61-070313 (NGK Insulators KK, published Apr. 11, 1986), and in French Patent No. FR 1,486,796 (Sangotoki Kabushiki Kaisha, Jun. 30, 1967).
One recent attempt at obviating the problems encountered with these or similar burner elements has been to construct burner elements from ceramic foams. Ceramic foams are made by soaking a polyurethane foam or other combustible foam material with a liquid ceramic material, drying off the mixture, and burning off the foam material, leaving a porous ceramic structure. The number of pores per inch in the resulting burner element can be selected by choosing the proper pore size of the precursor foam.
While ceramic foam materials were first developed for filtering high temperature casting alloys, the use of such ceramic foams as burner elements is described in the present Applicant's prior U.S. Pat. No. 4,900,245 (issued Feb. 13, 1990). Applicant's device as disclosed in that patent has found significant utility in devices such as commercial deep fat fryers. The burner element is made from a reticulated ceramic foam having a porosity of about 40 to about 100 pores per linear inch, formed about a perforate cylindrical metal diffuser. A high emissivity coating is placed on the reticulated ceramic foam burner element for substantially decreasing the likelihood of backflashing.
Applicant's prior burner element functions admirably for its intended purpose. However, its use in practice has been found to be subject to some drawbacks. Like other ceramic foam elements, some shrinkage and brittleness has been encountered. When the burner elements need to be replaced due to routine maintenance, moving, or unrelated repair of the fryers in which they are employed, the elements sometimes break because of this brittleness. Moreover, control of the pore size is perhaps not as precise as would be desired in order to insure that enough air is supplied to the combustion gas and avoid backflashing. Even when these problems are not encountered, the ceramic foam eventually melts down and becomes more brittle when the supply of air decreases, as it periodically may do. As a practical matter, once the ceramic foam elements are removed a single time from the device in which they are employed, they are often not subject to ready reuse.
Accordingly, it is an object of the present invention to provide a highly efficient radiant heat burner element for a fluid immersion apparatus or other device, which will uniformly burn combustion gases without backflashing.
It is another object of the present invention to provide a radiant heat burner element of lower shrinkage and lower brittleness than encountered with prior ceramic burner elements.
It is a further object of the present invention to provide a ceramic burner element which is readily subject to reuse and which does not easily break during replacement.
It is also yet another object of the present invention to provide a radiant heat burner element which is more reliable and less subject to clogging than have been past ceramic burners.